1. Field of the Invention
The present invention relates to an apparatus and method for amplifying a communication signal. More particularly, the present invention relates to an apparatus and method for improving the time alignment of signals used for amplifying a signal while reducing overall complexity.
2. Description of the Related Art
Portable terminals, which typically include a battery powered device that allows a user to wirelessly communicate with others, are used extensively in modern society. In this case, a portable terminal denotes not only any mobile unit that is typically carried by a user, but also a base station and other components that support the portable terminal. As the portable terminal continues to gain popularity, manufacturers continue to seek new ways in which to improve their product. One aspect to improve a portable terminal is to reduce its size and weight, thus making it more convenient to carry. Another aspect is to provide advanced features. However, reductions in size and weight are often limited by the power requirements necessary to provide the advanced features. That is, to supply the necessary power required for processing and supporting of the advanced features, the battery of the portable terminal must be adequately sized to ensure sufficient capacity for a reasonable amount of time. Thus, a reduction in the size of the portable terminal, due to the necessary size of the battery, is difficult to obtain.
Another consideration in battery sizing, and in power use in general, is the amount of power necessary to transmit a wireless signal. For transmission of the wireless signal, the portable terminal uses a Radio Frequency (RF) transmitter that includes a Power Amplifier (PA). The PA is used to amplify signals for outputting by the portable terminal's antenna. To maximize the efficiency of the PA, it is operated in its non-linear region, near saturation. However, due to the non-linearity of the PA, the Adjacent Channel Leakage Ratio (ACLR) becomes unacceptable since the output spectrum will expand and cause interference with adjacent transmission channels. To address this problem, an amplifier linearization technique may be performed by employing an adaptive Digital Pre-Distorter (DPD) to pre-process the baseband signal that is input into the PA so that the PA output behaves linearly.
FIG. 1 is a block diagram of a transmitter according to the prior art.
Referring to FIG. 1, the transmitter 100 includes a DPD 101, an envelope modulator 103, a Digital-to-Analog Converter (DAC) 105, an up-converter 107, a PA 109, an output unit 111, a down-converter 113, an Analog-to-Digital Converter (ADC) 115, and a Digital Signal Processor (DSP) 117.
The DPD 101 receives a baseband signal to be transmitted. The DPD 101 operates by pre-distorting the baseband signal. Combining the DPD 101 and the PA 109 will provide a substantially linear system since the DPD 101 acts as the inverse model of the PA 109.
The signal distorted by the DPD 101 is provided to the DAC 105 for conversion from a digital to an analog signal. The analog signal output from the DAC 105 is provided to the up-converter 107 for up-conversion and then provided to the PA 109 for amplification. The envelope modulator 103 is provided when Envelope Elimination and Restoration (EER) and Envelope Tracking (ET) type amplifiers are being used, otherwise there is no need for the envelope modulator 103 and no need for envelope time alignment between points D and C. In this case, only loop time alignment is needed between points A and B.
The signal amplified by the PA 109 is output to the output unit 111. Though not illustrated, the output unit 111 may include an antenna, a multiplexer, and the like for outputting and receiving signals. The signal output by the PA 109 is also fed back to the down-converter 113 which provides the down-converted signal to the ADC 115. The ADC 115 converts the down-converted analog signal to a digital signal and provides the digital feedback signal to the DSP 117 for processing.
To compute and constantly update the pre-distortion value provided by the DPD 101, the DSP 117 needs to capture two signals: the transmitted baseband or reference signal and the PA output or feedback signal. In addition, the reference and the feedback signals, illustrated respectively as points A and B in FIG. 1, must be time aligned with very high accuracy. That is, since the feedback signal is a delayed version of the input signal, the time delay between these two signals (i.e., the loopback delay) has to be estimated with very high accuracy in order to properly apply the pre-distortion. The transmitted baseband signal is digital prior to analog conversion by the DAC 105. Accordingly, processing of the baseband signal presents few problems for the DSP 117. However, the feedback signal supplied by the output unit 111 is initially in analog form. Thus, the feedback signal first needs to be down-converted by the down-converter 113 and converted to digital form by the ADC 115 before it can be processed by the DSP 117. This down-conversion and digitization, as well as the processes performed by the DAC 105, the up-converter 107, and the PA 109, are of significance because each process not only delays the feedback signal, but the delay may vary based on factors such as temperature, and the like.
Moreover, because the transmitter 100 of FIG. 1 employs the envelope modulator 103 to implement the ET or EER technique, an additional time alignment is required between an envelope signal output by the envelope modulator 103 and the up-converted baseband or loopback signal output by the up-converter 107. This envelope delay is illustrated in FIG. 1 as the delay between points C and D. This delay needs to be estimated in order to time align the signals with even higher accuracy than the loopback time alignment. That is, without high precision time envelope and loopback signal alignment respectively at points C and D and points A and B, the DSP 117 will not be able to compute the proper parameters for the DPD needed to linearize the amplifier.
Time alignment in the prior art consists mainly of over-sampling the signals and applying a cross-correlation technique. For example, in the transmitter illustrated in FIG. 1, the baseband and feedback signals at points A and B are over-sampled and analyzed by a cross-correlation technique in order to determine the delay between the two signals. Other techniques may include some type of Time Delay Estimation (TDE) in a closed loop fashion.
In the prior art, in order to achieve a high accuracy time alignment with a resolution 1/20 of a sample or better, the acquired signals must be over-sampled. Over-sampling is required prior to the cross-correlation operation in order to increase the accuracy of the time alignment. Typically, an over-sampling of 10 to 20 times is necessary to achieve sufficient results. However, because such a large over-sampling is required and because the cross-correlation techniques are mathematically intensive, these operations require a large amount memory and a powerful DSP, which increase the overall cost of the portable terminal. Moreover, such extensive computations and memory operations require power that must be supplied from the battery, which in turn limits the ability to reduce the battery size. All of these factors make it impractical, from the point of view of implementation and cost, to provide an adequate time alignment.
Another problem with these techniques is the difficulty in finding an optimum delay when using the computed cross-correlation. That is, determining the peak of a cross-correlation curve does not necessarily produce an optimum time delay estimation due to the analog noise and non-linear distortion of the feedback signal when such techniques are implemented. Moreover, the over-sampling by up to 20 times may not be enough to provide consistent results, which in turn leads to an inferior quality of linearization. Even further, it is very difficult to implement the scheme of over-sampling the signal followed by a cross-correlation operation in a Field Programmable Gate Array (FPGA) due to the limitation of the FPGA clock speed and memory. Therefore, because these techniques are not suited for fast, real time tracking of the time delay in order to make the proper time alignment, a need exists for an improved apparatus and method for providing improved time alignment in a transmitter.